1. Field of the Invention
The present invention relates to a driving apparatus for a cold cathode fluorescent lamp (CCFL). More particularly, the present invention relates to a driving apparatus for a CCFL capable of providing a high output voltage necessary for an open lamp and thereby satisfying an output standard required to drive the lamp.
2. Description of the Related Art
A cold cathode fluorescent lamp (CCFL) has fluorescent material coated onto its interior wall of a glass tube. The CCFL has electrodes attached to both ends of the tube, where tens of Torr of gas mixtures and a predetermined amount of mercury are sealed. The CCFL bears plenty of advantages such as high brightness, high color rendering, low power consumption of 2.0 W to 5.0 W, long useful life of 15,000 to 60,000 hr in a rated lamp current. Therefore it is well-utilized as a lighting device such as an LCD back-light. However, the CCFL exhibits large impedance if current is not applied and smaller impedance once current is applied and the lamp is ignited. A driving voltage necessary to start the CCFL is at least 1300V and the voltage after ignition is at least 800V.
Consequently, an output voltage standard should be at least 1300V in case of starting the CCFL. Also even in the open-lamp condition, a high voltage standard of at least 1300V should be outputted during a predetermined period of time.
FIG. 1 is a circuit diagram showing a driving apparatus for a CCFL according to the prior art. Referring to FIG. 1, a device for turning on/off the CCFL 10 comprises: an oscillator 11 for providing a reference clock; an PWM driver 12 for generating a PWM switching control signal having an on/off duty rate varied according to a predetermined cycle based on the reference clock provided by the oscillator 11; an inverter 13 for inverting an inputted current into alternating voltage, the inverter including two switching devices SW1, SW2 of a half-bridge structure which switch on and off alternately in response to the switching control signal outputted from the PWM driver 12; and a resonance circuit 14 for outputting the alternating current of a predetermined frequency inputted from the inverter 13 to a lamp.
A switching cycle of the inverter 13, or frequency of the outputted alternating current is fixed at ½ of the reference clock generated in the oscillator 11 by the PWM driver 12. However, the PWM controller 12 adjusts an on/off duty rate in response to the PWM control signal VPWM to control an outputted driving voltage and brightness of the lamp. In this case, as the switching signal has a longer “on” duration, the level of the voltage outputted to the lamp 10 becomes higher.
At this time, as shown in FIG. 1, the driving apparatus for the lamp of a half-bridge structure has a maximum duty rate of the switching control signal to be 50% or less. Thus, only control of a pulse width as stated above cannot satisfy the voltage standard for starting the lamp and the output voltage standard for an open lamp.